The invention relates in general to the synchronization of a radio receiver to a received signal. In particular the invention relates to the realization of symbol synchronization in a system wherein the received signal contains a certain guard interval the timing of which has to be right in the reception, so that potential multi-path components in the received signal can be utilized in an optimal manner.
Abbreviation OFDM (Orthogonal Frequency Division Multiplex) refers to a modulation method in which the transmitting apparatus divides and combines the transmitted signal into several subcarriers which are located on the frequency axis at regular intervals on a certain frequency band and which are sent simultaneously. Known radio-frequency communications systems that employ OFDM modulation include the DAB (Digital Audio Broadcasting) and DVB (Digital Video Broadcasting) systems. The former is specified in general outline in the standards drawn up by the European Broadcasting Union (EBU) and the European Telecommunications Standards Institute (ETSI), and the latter is specified in general outline in a draft standard by the same organizations. In these systems, a section of a digital signal to be transmitted on a certain subcarrier is encoded into phase and/or amplitude changes with respect to a certain known phase. That time slice of the transmitted signal during which the modulating phase state is constant separately at each subcarrier frequency is called an OFDM symbol, or a symbol in short.
Successful OFDM reception requires that the receiver maintains the correct symbol synchronization and sampling frequency. Symbol synchronization means that the receiver knows at which point in time each symbol begins and times the symbol detection correspondingly. Sampling frequency refers here to the frequency at which the A/D converter in the receiver takes samples from the received analog oscillation in order to convert the signal into digital form, whereby the A/D converter and subsequent circuits can interpret to which bits or bit combinations in the digital data flow the signal phase changes refer. In addition, the receiver has to maintain frequency synchronization, i.e. tune the reception and mixing circuits so that the detected frequency band covers all subcarriers of the OFDM signal at an accuracy which is less than half of the difference between two adjacent subcarriers. Maintaining the symbol synchronization, sampling frequency and frequency synchronization is especially difficult if the transmitter and receiver are moving with respect to each other. The receiver may be located in a car, for example, and as the car moves around in an urban environment, the propagation path of the radio signal changes constantly, resulting in attenuation and reflections. The receiver may also be located in a satellite, and as the satellite moves, the speed difference between the receiver and the satellite changes, being possibly up to several kilometres per second. This patent application is especially concerned with achieving and maintaining symbol synchronization.
An adjustment method for symbol synchronization and sampling frequency in an apparatus receiving OFDM-modulated transmissions as well as an apparatus realizing such a method is known from Finnish patent application Ser. No. 963649. The method disclosed is based on utilizing time-domain correlation characteristics of the reference signal in an OFDM transmission. In the DAB system, the reference signal means a phase reference symbol, and cross-correlation between the received format and the known format of that symbol yields the instantaneous impulse response. In the DVB system, the impulse response is estimated from scattered pilot subcarriers for four consecutive symbols. The required changes in the symbol synchronization and sampling frequency can be deduced by monitoring how the impulse response changes from one measurement to another. The symbol synchronization is preferably set such that the guard interval between the symbols coincides with the beginning of the correlation function representing the impulse response. A sampling frequency error shows between the measurements as a slow and monotonously continuous shift of the maximum of the correlation function representing the impulse response. By correcting the sampling frequency the receiver attempts to eliminate said change.
From publication xe2x80x9cLow-Complex Frame Synchronization in OFDM Systemsxe2x80x9d by J-J. van de Beek, M. Sandell, M. Isaksson, P.O. Bxc3x6rjesson, IEEE International Conference on Universal Personal Communications, Tokyo 1995, a method for achieving symbol synchronization by utilizing characteristics of data transmitted in an OFDM system is known. This method is briefly explained below. FIG. 1 shows a simple OFDM system model wherein complex numbers Xk, kxcex5[1,N] taken from a fundamental set, or constellation, are to be transmitted (cf. allowed points in phase-amplitude coordinate system in quadrature amplitude modulation, QAM). The complex numbers xk are used for modulating N subcarriers by means of an inverse discrete Fourier transform (IDFT) in block 101. The result is N samples s, the last L of which are copied to the beginning of the sample set. After the copying, the number of samples is N+L and a given sample can be marked sk, where kxcex5[1,N+L]. The samples copied to the beginning of the sample set constitute a so-called guard interval because in time domain they appear as a period in the beginning of the symbol the contents of which are a copy of the end of the symbol.
A parallel-to-serial converter 102 is used to generate an OFDM symbol, marked s(k). When traveling from the transmitter to a receiver through a given channel the symbol s(k) is affected by the impulse response h(k) of the channel and noise n(k) is added to it. The receiver sees the received sample sequence, marked r(k). The latter undergoes a serial-to-parallel conversion in block 103 producing samples rk, where still kxcex5[1,N+L]. Only the last N samples are independent of each other, so they are taken to block 104 where a discrete Fourier transform takes place. Symbol synchronization is the same as determining out from which location in the received sample sequence said last N samples will be taken. The end result are complex numbers yk, kxcex5[1,N]. If reception was fully successful, those complex numbers are the same as the transmitted complex numbers xk.
In said method, a copy r(kxe2x88x92N) is made of the received symbol r(k) and said copy is delayed by N samples with respect to the original received symbol. A correlation function is defined between the copy and the original:
j(k)=r(k)r*(kxe2x88x92N)xe2x80x83xe2x80x83(1)
where * stands for complex conjugation. Then we can calculate the moving sum using a window of L samples                               u          ⁡                      (            k            )                          =                              ∑                          i              =              0                                      L              -              1                                ⁢                      j            ⁡                          (                              k                -                1                            )                                                          (        2        )            
and sliding the window over the received sample sequence for the length of 2N+L samples. FIG. 2 shows a received sample sequence r(k) with a guard interval 201 for a symbol and the corresponding period 202 at the end of the symbol, a copy r(kxe2x88x92N) of the received sample sequence, sequence portions 201xe2x80x2 and 202xe2x80x2 corresponding to the guard interval 201 and original samples 202, and a correlation result j(k) with a period 203 that represents high correlation. In addition, FIG. 2 shows the value of the moving sum u(k) in such a way that the value on the u(k) curve corresponds to the sum according to equation 2 in that window the right edge of which coincides with the value in question. The figure shows that the u(k) curve has a distinct correlation peak 204 the top of which coincides with the end of a given symbol in the original sample sequence. In said method according to the prior art symbol synchronization is based on the detection of the top of the correlation peak 204.
The methods according to the prior art described above are applicable in cases where the signal""s delay spread is small, i.e. all significant components of the signal caused by multipath propagation are located relatively close to each other in time. From prior art an efficient method to maintain symbol synchronization in an OFDM receiver when the delay spread is large is not known.
An object of this invention is to provide a method and apparatus for achieving and maintaining symbol synchronization in an apparatus receiving OFDM-modulated transmissions. It is particularly an object of the invention that the method according to the invention does not require unreasonable computing capacity nor hard-to-manufacture special components, and that the apparatus realizing the method be suitable to large-scale series production as regards its manufacturing costs.
The objects of the invention are achieved by computing in the receiver a correlation between two mutually delayed sample sequences and, after that, a correlation between two correlation results computed that way.
The invention is directed to a method for achieving and maintaining symbol synchronization in a receiver, comprising steps in which
the correlation between an undelayed sample sequence representing a received signal and a delayed sample sequence representing a received signal is calculated, and
a moving sum is calculated of the calculated correlation to produce a sequence of values. In addition, the invention is directed to a receiver apparatus for realizing said method.
The method according to the invention is characterized in that it comprises steps in which
values in said value sequence are multiplied by predetermined other values obtained from said value sequence in order to produce a modified sequence of values, and
a moving sum is calculated of the modified sequence of values in order to produce a peak value that indicates the correct symbol synchronization.
The receiver according to the invention is characterized in that it comprises means
to multiply values in said value sequence by predetermined other values obtained from said value sequence in order to produce a modified sequence of values, and
to calculate a moving sum of the modified sequence of values in order to produce a peak value that indicates the correct symbol synchronization.
The weaknesses of the methods according to the prior art become apparent especially when noise is strong (signal-to-noise ratio is poor) and/or multipath propagation causes the channel impulse response to become distributed over a time period comparable to the length of the guard interval. Due to the noise, a correlation peak calculated according to the prior art will not be distinct and the multipath propagation may result in several correlation results from among which it is hard for the receiver to choose the correct timing point. The double correlation according to the invention firstly reduces the effect of noise on correlation peaks. Secondly, the method according to the invention can affect symbol synchronization so that most of the power of the impulse response is within the guard interval timed according to the invention, so that the receiver has the best chances to utilize the energy of the strongest multipath components.
In the method according to the invention a correlation and moving sum between the original sample sequence and a sample sequence that has been delayed for a number of samples equalling the length of the information contents proper is calculated in a known manner. Then a correlation and absolute value of the moving sum between the correlation result thus obtained and a correlation result delayed for a number of samples equalling the length of the whole symbol is calculated. The latter correlation calculation yields a correlation peak indicating the point where most of the channel impulse response power is within the summing window used in the calculation of the moving sum. That correlation peak is used to achieve and maintain symbol synchronization.
The method according to the invention can be complemented by a procedure that compensates for a frequency error caused by a non-ideally set Fourier transform window. This is accomplished using a complex multiplier that equalizes the frequency errors of all symbols so that the constant frequency error can be compensated for using known channel equalization.